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More Redirection

CS155 Redirection

Streams

• When a Unix program is started a set of associated streams are opened.
• stdin – standard input
• stdout – standard output
• stderr – standard error
• These streams allow us to manipulate the input and output of a Unix program.

Streams and Redirection

• We have already seen how to send data to stdin using <
• Also, we can attach stdout to a file or program using >, >>, or |
• Error messages go to stderr, not stdout.

Redirecting stderr and stdout

% ls -l
-rw-rw-r-- 1 applin fac 1234 Jan 17 14:26 alpha
-rwxr--r-- 1 applin fac 4918 Jan 15 12:23 beta
% rmdir alpha > output
rmdir: ‘alpha’: Not a directory
%

Why didn’t the error message go to the file output?

Redirect just stdout with > or just stderr with 2> or both stdout and stderr with >&.

Examples

% ls -l gamma delta
-rwxr--r-- 1 applin fac 4918 Jan 15 12:23 gamma
ls: delta: No such file or directory
% ls -l gamma delta >output
ls: delta: No such file or directory
% ls -l gamma delta >&output
%